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Proved reserves (Details 5) (USD $) In Millions, unless otherwise specified | Dec. 31, 2013 MBoe | Dec. 31, 2012 MBoe | Dec. 31, 2011 MBoe
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Proved Developed and Undeveloped Reserves [Abstract] |  |  |
Proved developed reserves, energy | 66,140 | 69,131 | 79,460
Proved undeveloped reserves, energy | 14,555 | 11,355 | 18,191
Balance at end of year, energy | 80,695 | 80,486 | 97,651
Changes in proved and undeveloped reserves [Abstract] |  |  |
Change in PUD reserve | 3,200 |  |
Negative revision to PUD reserves | (1,600) |  |
New PUD reserves | 11,900 |  |
Converting PUD reserves | 7,100 |  |
Drilling, completion and future development costs [Abstract] |  |  |
Drilling and completion capital | $ 127.3 |  |
Future development costs estimated to be spent to develop PUD reserves year one | 143.6 |  |
Future development costs estimated to be spent to develop PUD reserves year two | 116.0 |  |
Future development costs estimated to be spent to develop PUD reserves year three | $ 18.1 |  |
Oil (MBbls) |  |  |
Proved Developed and Undeveloped Reserves [Abstract] |  |  |
Proved developed reserves, volume | 31,394 | 27,412 | 23,653
Proved undeveloped reserves, volume | 9,625 | 6,041 | 3,352
Balance at end of year, volume | 41,019 | 33,453 | 27,005
NGL (MBbls) |  |  |
Proved Developed and Undeveloped Reserves [Abstract] |  |  |
Proved developed reserves, volume | 5,322 | 5,342 | 5,225
Proved undeveloped reserves, volume | 1,280 | 1,811 | 2,117
Balance at end of year, volume | 6,602 | 7,153 | 7,342
Natural Gas (MMcf) |  |  |
Proved Developed and Undeveloped Reserves [Abstract] |  |  |
Proved developed reserves, volume | 176,546 | 218,259 | 303,495
Proved undeveloped reserves, volume | 21,899 | 21,019 | 76,332
Balance at end of year, volume | 198,445 | 239,278 | 379,827

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- Definition Change in PUD reserve. + References No definition available. + Details
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- Details
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- Definition Converting PUD reserves. + References No definition available. + Details
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- Definition Drilling and completion capital required to convert PUD reserves into proved developed reserves. + References No definition available. + Details
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- Definition Future development costs estimated to be spent to develop PUD reserves year one. + References No definition available. + Details
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- Definition Future development costs estimated to be spent to develop PUD reserves year three. + References No definition available. + Details
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- Definition Future development costs estimated to be spent to develop PUD reserves year two. + References No definition available. + Details
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- Definition Amount of negative revision to PUD reserves. + References No definition available. + Details
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- Definition Amount of new PUD reserves. + References No definition available. + Details
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- Definition The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable. + References No definition available. + Details
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